TEPCO said water samples collected from a well on Thursday contained 400,000 becquerels per litre of beta-ray emitting substances, the highest reading since the nuclear accident was triggered in March 2011.

Samples in previous days had been showing levels of just 60 to 90 becquerels per litre.

The reading for radioactive tritium also shot up to a record high of 790,000 becquerels, TEPCO said.

The leaking storage tank was discovered in August and TEPCO said measures taken since then had included transferring water from the tank and removing the surrounding soil.

"Other wells are not showing such a high level. We need to take additional measures for the areas around this well," Masayuki Ono, TEPCO spokesman, told a press conference on Friday evening.

TEPCO has removed most of the soil contaminated by the August spill, but might have failed to collect some radioactive materials, Ono said.

Heavy rain over recent weeks could have washed contaminated materials into the well, he added, saying TEPCO would remove the toxic soil and groundwater.

A huge earthquake and tsunami in 2011 devastated Japan's northeast coast and sparked reactor meltdowns at the Fukushima plant by knocking out its cooling systems.

TEPCO, which has come in for growing criticism over its handling of the aftermath of the disaster, has poured thousands of tonnes of water onto the reactors to keep them cool in an ongoing operation.

Radioactive water is being stored in around 1,000 tanks but the company has admitted contaminated liquid has made its way into the sea.